1. Field of the Invention
The present invention relates to pulpers for a fiber suspension, and, more particularly, to pulpers having a screen plate for separating good fibers from effluent.
2. Description of the Related Art
A pulper is used for making fibers which are used in a fiber suspension in a paper-making machine. The source of fiber may be, e.g., wood fiber in the form of virgin fiber or recycled fiber. The pulper grinds the source of fiber into individual fibers which are used in the fiber suspension in a paper-making machine.
It is known to use a pulper with an extraction/screen plate and a rotor which are disposed within a vessel. For simplicity, the extraction/screen plate is referred to as a screen plate herein. The rotor causes the source of fiber to rotate past the screen plate. Good fiber passes through a plurality of holes in the screen plate for further processing, while undesirable substances such as foreign matter, etc., do not pass through the holes in a screen plate and may be discarded.
A screen plate is typically perforated or has other profiled perforations or slots. The surfaces of the screen plate are either flat or have various forms of surface projections, e.g., bars, strips, etc. The stationary projections work in conjunction with the leading edges of the rotating rotor vanes to break down flakes/knits and other undefibered materials in a slurry.
Strips and bars on a screen plate have the disadvantage that they reduce the available screening surface area and are therefore restricted in quantities to maintain the necessary open screening area/hydraulic through-put capacities. The process of applying the strips and bars typically requires high levels of heat (e.g., welding) resulting in warping and surface distortion to the plate, requiring additional work processes to return the plate surface to an acceptable level of flatness.
A screen plate may include holes which are formed directly in the screen plate, or the holes may be defined by inserts which are received within corresponding openings in the screen plate. The inserts may be removably or irremovably attached to the base plate of the screen plate, such as by arc welding. The inserts may extend above the level of the base plate, and the exposed edges of the inserts exert a shearing action on the fiber suspension. An example of a screen plate including inserts extending above a level of the base plate is disclosed in U.S. Pat. No. 6,254,729 (Doelle et al.) which is assigned to the assignee of the present invention.
For ease of manufacturing, inserts as described above are typically formed as short tube sections which are inserted into corresponding circular holes formed in the base plate. The exposed edges of the inserts thus provide a rounded leading edge which limits the amount of shearing action induced on the fiber suspension. Further, the time necessary to metalurgically bond the inserts to the base plate, such as by welding, increases the manufacturing costs of the screen plate. Moreover, material tends to accumulate on the screen plate in the area between the inserts since there are no holes in the base plate, other than the larger diameter openings in which the inserts are received.
What is needed in the art is a screen plate which provides a maximum available edge impacting surface profile without reducing the available screen plate area.